Indoles are useful as starting materials for tryptophan, aromatics, medicines, agricaltural chemicals, stabilizers for polymers, and the like. Various methods have been known for preparation of indoles by a vapor phase reaction. For example, Japanese Patent Publication (unexamined) No. 78163/1973 discloses that indoles are prepared by contacting 2-alkylanilines at 500.degree. to 750.degree. C. with an iron oxide catalyst containing an alkali metal compound. Japanese Patent Publication (unexamined) No. 57966/1973 discloses that indoles are prepared by contacting o-nitroalkylbenzene at 300.degree. to 550.degree. C. with a catalyst containing ruthenium, palladium or platinum. Further, Japanese Patent Publication (unexamined) No. 65266/1977 discloses that indoles are prepared by contacting phenylhydrozines and a carbonyl compound at 300.degree. C. with a Lewis acid (e.g., zinic chloride). Furthermore, Chemical Abstracts, Vol. 62 (1965), 16174h discloses that indole are prepared by contacting aniline and ethanol with an alumina catalyst or an iron oxide-alumina catalyst. Additionally, Chemical Abstracts, Vol. 63 (1965), 11477 g discloses that indole are prepared by contacting aniline and acetylene at 620.degree. to 700.degree. C. with an iron oxide-alumina catalyst. However, the methods described in Japanese Patent Publication (unexamined) No. 78163/1973 and Chemical Abstracts, Vol. 63 (1965), 11477 g are disadvantageous in that the vapor phase reaction must be carried out at a temperature higher than 500.degree. C. In the method described in Japanese Patent Publication (unexamined) No. 57966/1973 it is necessary to use an expensive noble metal catalyst, and the methods described in Japanese Patent Publication (unexamined) Nos. 57966/1973 and 65266/1977 require the use of expensive o-nitro-alkylbenzene or phenylhydrazines as the starting material. Further, the methods described in Japanese Patent Publication (unexamined) Nos. 78163/1973 and 57966/1973, Chemical Abstracts, Vol. 62 (1965), 16174h an ibid. Vol. 63 (1965), 11477 g are disadvantageous in that N-substituted indoles can not be prepared directly.